It is already known to use as a molding material compositions formed from a mixture of unsaturated polyester, a thermoplastic polymer, a monomer substance such as styrene to which is added a mineral filler, a catalyst, a stabilizer and a releasing agent, which exhibit a very slight, even zero, shrinkage after polymerization. The polyesters used to obtain this result are generally slightly saturated.
In numerous applications, the objects made from these compositions of polymerizable resins should exhibit good mechanical properties, in particular, good impact resistance. To do this, it is known to mix with these compositions cut yarns which most often are glass yarns.
These glass yarns are obtained by combining a large number of filaments, mechanically drawn from molten glass flowing from orifices made at the base of a spinneret heated by Joule effect. During drawing, these filaments are coated with a lubricant then gathered to give rise to a yarn exhibiting a certain cohesion. This continuous yarn is then cut into sections from a few millimeters to several dozen millimeters in length.
When the filaments are coated with a lubricant soluble in the composition of resins, the cut yarn introduced into said composition disintegrates and is dispersed in the filament state when the unit is mixed. On the other hand, if the filaments are coated with an insoluble lubricant the cut yarn introduced into the composition of resins remains integral, even at the end of the mixing operation.
In the first case, the element for reinforcing the final product is essentially the glass filament, in the second case it is the yarn formed from filaments that have remained glued to one another.
Depending on which type of reinforcement is involved, the flow of the mixture in the mold, and the degree of orientation of the glass fibers, the resulting mechanical properties and surface condition of the molded object can be varied over a wide range.
Thus, soluble lubricants are known for imparting to the object an excellent surface condition, a high tensile strength but a relatively mediocre impact resistance. The insoluble lubricants are well known for improving the impact resistance but to the detriment of the surface and tensile strength.
Taking into consideration the numerous factors that affect the qualities of the final product, one skilled in the art faces different situations which require, for example, to choose a better aptitude for molding to the detriment of physical properties, or to opt for an excellent surface condition to the detriment of the impact resistance and vice versa.
In a particular case, one skilled in the art has succeeded in reconciling a good aptitude for molding, a high level of mechanical properties and an excellent surface condition of the final product. There is involved a process of producing plates of thermoplastic resins reinforced with glass fiber, described by the British Patent Specification No. 1,367,960.
This process consists in continuously making a mat of cut yarns, relatively long and remaining integral, on which is deposited a layer of much shorter cut yarns and greatly disintegrated to the filament state. The unit is simultaneously heated and compressed, which has the effect of causing a part of the short fibers in the filament state to penetrate between the long fibers remaining in the yarn state.
Although this patent discloses a particular association of fibers of different lengths, it does not suggest any valid way of obtaining a homogeneous mixture of resin and fibers of different lengths. Actually, this patent does not suggest any solution for obtaining such a mixture, while avoiding breaking of the fibers (especially the longer ones) caused by the shearing forces, which inevitably intervene during the kneading operation and during the molding operations, particularly when injection molding is performed.